The present invention relates to an electrochemical cell and, more specifically, to an anode-limited balanced cell that is particularly useful in implantable medical devices such as cardiac defibrillators.
Implantable cardiac defibrillators are used to treat patients suffering from ventricular fibrillation, a chaotic heart rhythm that can quickly result in death if not corrected. In operation, the defibrillator device continuously monitors the electrical activity of the heart of the patient, detects ventricular fibrillation, and in response to that detection, delivers appropriate shocks to restore a normal heart rhythm. Shocks as large as 30-35 Joules may be needed. Shocks are delivered from capacitors capable of providing that energy to the patient in a fraction of a second. In order to provide timely therapy to the patient after the detection of ventricular fibrillation, it is necessary to charge the capacitors with the required amount of energy within only a few seconds. Thus, the defibrillator power source must have a high rate capability to provide the necessary charge to the capacitors. It must also possess low self-discharge in order to have a useful life of many months, and it must be highly reliable to provide an urgently needed therapy whenever necessary. In addition, since cardiac defibrillators are implanted, the battery must be able to supply energy from a minimum packaged volume.
Batteries or cells are volumetrically constrained systems. The size or volume of components that go into a battery (cathode, anode, separator, current collectors, electrolyte, etc.) cannot exceed the available volume of the battery case. In addition, the appropriate amount of some components depends on the amount of other components that are used. These components must be xe2x80x9cbalancedxe2x80x9d to provide discharge to the extent desired.
In a conventional cathode limited battery such as a lithium-silver vanadium oxide (LiSVO) battery commonly used in a defibrillator application, the capacity (Q+) of the cathode must not exceed the capacity (Qxe2x88x92) of the anode. Cathode limited cells have been used in battery powered implantable medical devices such as heart pacemakers because of the proven reliability of their discharge over the long periods during which they are implanted. The volume occupied by the other battery components also depends on the cathode capacity (Q+) as reflected by the amount of cathode material in the battery. The amount of electrolyte depends on the amount of cathode material and the amount of it to be discharged since the cathode material swells as the battery is discharged and requires more electrolyte to fill the additional cathode volume. The volume of the separator and current collector depends on the area of the electrodes. The area of the electrodes depends on the area required for consistent pulse output as the battery is discharged. All of these components must be adjusted for a given battery volume.
One method for constructing a lithium anode cell is to wind cathode and anode elements together to form a generally cylindrical or oblong coil. In a coiled design, anode material is available on either side of the cathode windings and will deplete into the cathode as the battery is discharged. Reliable performance is assured by having an excess of anode capacity beyond the usable capacity desired of the cathode. This excess lithium is distributed through the length of the anode. The lithium winding forming the outermost turn of the coil has cathode material adjacent to its inner circumference but not on the outer circumference. Therefore, the outermost turn of the anode need only be half the thickness of the inner turns. If an anode is constructed from a single piece of foil that is of uniform thickness, there is an additional excess of lithium on the outermost turn of the coil.
One limitation of a cathode-limited cell is that the excess lithium in the cell may reduce other materials after the cathode is fully discharged, which can lead to cell swelling. In order to prevent damage to circuitry surrounding the cell due to cell swelling, a reinforcing stainless steel plate is typically provided against the cell. This stainless steel plate occupies valuable space in an implantable device and adds weight to the overall device. In regard to implantable medical devices, a reduced size and weight is desirable to ease the implant procedure and avoid patient discomfort at the implant site. Eliminating the excess lithium or the need for the stainless steel plate would allow the cell size, and therefore the overall size of the medical device, to be reduced. Alternatively, the volume occupied by excess lithium and the steel plate could otherwise be taken up by cathode material to increase the battery capacity.
Another limitation of a cathode-limited cell is that its resistance increases as a function of time after the cell is discharged to the second voltage plateau on its discharge curve. By limiting the amount of lithium and electrolyte material in the cell, the cell may be designed to utilize only the first voltage plateau. Superior long-term cell performance can be achieved since the same useful capacity can be provided in a defibrillator application as in a conventionally balanced cell but at a higher voltage toward the end of the discharge cycle. The average capacitor charge time is shortened toward the end of the cell""s useful life, maintaining a more constant charge time throughout the life of the defibrillator. A lithium-limited balanced cell having these advantages is disclosed in U.S. Pat. No. 5,458,997 to Crespi et al, incorporated herein by reference in its entirety.
A method for manufacturing an anode-limited cell is needed that eliminates the excess anode material associated with a conventional coiled electrode. A prior art method for assembling a lithium anode to provide a thinner outer winding in a coiled electrode involves layering two lithium foil pieces. One lithium foil is provided long enough to form all of the windings in a coil, and the other lithium foil is provided long enough to form only the inner windings. When the two foils are overlaid, with an optional current collector sandwiched between, and coiled with a cathode, the inner coil windings are formed by a double layer of lithium foil and the outermost winding by a single layer of lithium foil.
This method, however, has several limitations. One limitation is the difficulty in handling two, long, thin pieces of lithium foil during the manufacturing process. Another limitation is tolerance xe2x80x9cstack-upxe2x80x9d. A given tolerance is associated with each dimension of manufactured lithium foil. When two layers of lithium foil are pressed together to form the length of the anode that becomes the inner windings of a coiled electrode, tolerance xe2x80x9cstack-upxe2x80x9d occurs, widening the total tolerance. For example, if the tolerance for each foil is xc2x110%, the total tolerance of the layered foils may be doubled to as much as xc2x120%. This wide tolerance may be unacceptable in an efficient anode-limited cell. Excess lithium takes up volume that could be devoted to further increasing the capacity of the cell or may lead to undesirable discharge characteristics and cell swelling. A lack of lithium could lead to premature battery depletion due to the cathode discharge becoming excessively limited by the amount of lithium available. Minimizing the tolerance of lithium and the total amount of excess lithium is important for at least two reasons: 1) the minimum amount of lithium, excluding the excess, must be maintained throughout the first voltage plateau to have reliable cell performance, and 2) the maximum amount of lithium including the excess must be consumed before the end of the second voltage plateau to avoid swelling.
Accordingly, a method for manufacturing a lithium anode is needed, therefore, that maintains a narrow tolerance of the lithium anode dimensions or mass such that the advantages of the discharge characteristics of an anode-limited cell may be realized.
The present invention addresses the problem of excess lithium in an electrochemical cell. One aspect of the present invention is a method for manufacturing an electrode assembly that allows a narrow tolerance of the anode material in order to achieve the improved discharge characteristics of an anode-limited cell. Other aspects of the present invention include providing an anode-limited cell with improved capacity or reduced overall size, avoiding premature battery depletion due to an insufficient amount of anode, and avoiding cell swelling.
These aspects of the invention are realized by providing an anode constructed from a thin piece of lithium foil joined at one end to a thicker piece of lithium foil such that the thin lithium foil forms the outermost winding of a coiled electrode and the thicker lithium foil forms the inner windings. The thicker foil forming the inner windings provides enough lithium for depleting into the cathode material facing both sides of the inner windings. The thinner foil forming the outermost winding provides enough lithium for depleting into the cathode material facing only the inner side of the outermost winding. The two lithium foils overlap each other to provide continuity, but this overlap is minimized to prevent an excess of lithium and reduce the amount of lithium required for construction. A metal grid functions as a current collector and advantageously stabilizes and reinforces the cohesive bond between the two lithium foils.
By using a thin lithium foil for the outermost turn, space is made available for additional cathode material such that the charge capacity of the anode-limited cell may be increased. Alternatively, the overall battery size may be reduced by limiting the amount of lithium. Moreover, cell swelling is avoided in a anode-limited cell, eliminating the need for a reinforcing steel plate, freeing even more space for cathode material or a reduction in the overall battery size.
Maintaining a narrow tolerance of the lithium anode dimensions is possible since the lithium anode primarily comprises a single layer of lithium foil with only a limited area of overlapping layers. Tolerance stack-up is avoided. Thus, the improved discharge characteristics of an anode-limited cell can be realized by implementing methods included in the present invention for assembling a lithium anode.